2288
P.
Neta and H. Levanon
Spectrophotometric Study of the Radicals Produced by the Reduction of syn- and anti-Azobenzene' P. Neta" and Halm Levanon* Radiation Laboratory and Chemistry Department, University of Notre Dame, Notre Dame, Indiana 46556 (Received June 6, 1977) Publication costs assisted by the Division of Basic Energy Sciences, U.S.Department of Energy
The radicals produced by one-electron reduction of syn- and anti-azobenzene,4 and A,, in various media were studied by spectrophotometric pulse radiolysis. In aqueous solutions both forms of A are reduced by eaq-( k = 3 x 1O1O M1 s-l) and by various alcohol radicals ( ~ A + ( c H ~ ) ~ c o H= 4 X lo8 and ~ A + ( c H ~ )=~ 2c oX- lo9 M-' s-l1. Similar reactions are observed in neat i-PrOH. The transient spectra exhibit an intense absorption in the 35C-400-nm region p d a weaker one at 550-600 nm. Shifts of the maxima with pH are attributed to the acid-base equilibria AH2+P AH P A- with pK values of 2.9 and 13.7, respectively, in aqueous solutions. In THF solutions the main peak of A- is at 430 nm. All the kinetic and spectral parameters are identical whether one starts with the syn or anti isome;. Product analysis shows radiolytic conversion only in the direction A, A,. In MTHF glasses the spectra of 4-and A, are different, the latter being an order of magnitude more intense. It is concluded that in solution at room temperature A; is rapidly converted into A,. +
Introduction It has been recently demonstrated that the anion radicals of cis- and trans-stilbene, S, differ in their physicochemical properties such as optical and ESR absorption spectra and protonation rates in alcoholic solut i o n ~ . These ~ ! ~ observations led to the quantitative study of the electron transfer
si t stz s,
t it-
(1)
and the direct isomerization3
si 2 8,
(2)
The kinetics and equilibrium of reaction 1were determined by optical pulse radiolysis experiments4whereas reaction 2 is too slow to be monitored under those conditions. In the present study we searched for similar phenomena in the analogous compounds syn- and anti-azobenzene (A). In general, it is expected that isomerization around an N=N bond would be more facile than that around a C=C bonde5The results presented here show that the reaction A,
+
A,
Results a n d Discussion Transient Spectra in Aqueous Solutions. Preliminary experiments with azobenzene were carried out in neutral aqueous solutions containing 2-propanol as a scavenger for OH and H. A transient spectrum with maxima around 350 and 550 nm was observed. The kinetics of formation of this spectrum consisted of two steps (Figure 1): an initial build up attributed to rapid reaction with the hydrated electron
(3)
is very fast and no experimental evidence was found to indicate a reaction in the opposite direction. Consequently, the electron transfer reaction similar to (1) becomes negligible. Experiments in protic solvents showed that A- can protonate rapidly in two consecutive steps and the corresponding pK values were determined. Experimental Section The two isomers of azobenzene where prepared photochemically from a commercial sample as described in the literature6 and were identified Fresh solutions were prepared immediately before each experiment and their absorption spectra were recorded to confirm that no thermal isomerization took place. The alcohols and the inorganic compounds were Baker Analyzed reagents. THF and MTHF were dried and distilled under vacuum over K-Na alloy.g For large quantities, THF was refluxed over LiA1H4and freshly distilled under a stream of dry nitrogen for each experiment. Water was The Journal of Physical Chemistty, Vol. 81, No. 24, 1977
purified by passage through two Millipore systems, first the Milli-RO and then the Milli-Q, The half-life of hydrated electrons in this water, containing 2-methyl-2propanol at pH 11,was found to be greater than 40 p s a t dose rates similar to those used in the present experiment. The computer controlled pulse radiolysis apparatus was that described previouslylOJ1except for the radiation source which in this case was an ARC0 LP-7 linear accelerator. Pulses of 5-ns duration were used and the total radical concentration produced by each pulse was in the range of 2-4 pM. Pulse conductometric experiments were carried out using the apparatus described previously.12
(4)
followed by the slower electron transfer from the 2propanol radical A t (cH,),COH-
k,
A-t (CH,),CO
+ H+
(5)
In order to confirm that the same radical is produced in both reactions 4 and 5 the transient spectrum was recorded at different times. The spectrum observed after completion of reaction 5 (80-100 p s after the pulse) was found to be identical with that recorded 1p s after the pulse, i.e., when only reaction 4 is complete. Further confirmation was obtained from experiments in the presence of 2-methyl-2-propanol instead of i-PrOH, where reaction 5 is eliminated, In this case the formation of the transient consisted of the rapid reaction 4 only. The spectra recorded 1p s after the pulse using solutions of A, and A, are shown in Figure 2. It is clearly seen that the spectra obtained from both isomers are identical, suggesting that the structure of the radical is common for both starting materials. It is also seen in Figure 2 that the spectra in
-
-
2289
Radicals Produced by Reduction of syn- and anti-Azobenzene
1 ( IO pa / divirion )
Flgure 1. Formation of the azobenzene transient absorption in aqueous M, [i-PrOH] = 1 M, solution containing 2-propanol: [A,] = 1 X pH 7, X 350 nm.
I
I
300
I
I
400
xx)
I
6w
I
700
A(nrn)
Flgure 3. Absorption spectra observed with azobenzene in aqueous solutions containing 2-propanol. The spectra were recorded 80-100 fis after the pulse, after reaction 5 was complete: [A] = 1 x M, [I-PrOH] = 1 M; (0) 1 M NaOH; (A) pH 7; (0)pH 1. Extinction coefflcients were calculated using G = 6, equal to that used in the thiocyanate dosimetry. The insert shows the effect of pH on the transient absorption at 390 nm. The best-fit curve was calculated using pK1 = 2.9 and pK2 = 13.7.
values are assigned to the protonation of the azobenzene anion radical in two successive steps (C,H,NHNHC,H,)+.
;f
C,H,NHhC,H,
pK 2.9
AH; (C,H,N= NC,H,)-*
k (nm)
Flgure 2. Absorption spectra observed N 1 ps after the pulse with syn- and anti-azobenzene in aqueous solutions containig 2-methyl2-propanol: [A] = 1 X M, [t-BuOH] = 1 M; (0)A,, 1 M NaOH; (0)A,, 1 M NaOH; (A) A,, pH 7, (A) A,, pH 7. Extinction coefficients were calculated using G = 2.8 and thiocyanate dosimetry. The difference between the A and A data points below 360 nm results from the difference in the parent compound absorption; the dashed line is the corrected curve for both isomers.
neutral and alkaline solutions are different. This finding suggests that A- protonates rapidly in neutral solution. To examine the acid-base equilibria of the azobenzene radical it is advantageous to be able to cover the whole pH range. Solutions containing t-BuOH are only useful at pH > 4 since the conversion of e,; into H in acid solution may result in the formation of a different radical from azobenzene (see below). Experiments were, therefore, carried out in the presence of i-PrOH, where azobenzene is expected to undergo one-electron reduction at all pH values. Typical spectra are shown in Figure 3. At pH 14 the spectrum is identical with that observed in the presence of t-BuOH (Figure 2),13confirming that the reaction of the (CH3)&OH radical with azobenzene is an electron transfer process. The spectra taken in neutral solutions containing either i-PrOH (Figure 3) or t-BuOH (Figure 2) differ only slightly, i.e., the absorption in the 400-500-nm region is slightly higher in t-BuOH than in i-PrOH solutions. This difference is attributed to the contribution of the absorption of the H adduct to A, which is formed only in t-BuOH. This assignment was also confirmed by experiment with solutions containing both t-BuOH and N20 so that A can react with H atoms only. The spectrum of the transient at pH 1was recorded with i-PrOH solutions only. As stated above, the reaction of the H atoms with azobenzene does not necessarily lead to the same species, but may result in substantial addition to the rings. The pH dependence of the absorption in the presence of i-PrOH is shown in the insert of Figure 3. From the results pK values of 2.9 and 13.7 were determined. These
t pK 13.7
AH
AIn order to verify these protonation steps, a pulse conductometric experiment was carried out. The irradiation of an unbuffered neutral aqueous solution containing 1X lo4 M azobenzene and 0.5 M t-BuOH resulted in practically no net change in conductance. (The change -10 ps after the pulse was 13% of the conductance measured after irradiation of a reference solution containing CH3C1.) It is, therefore, concluded that in neutral solution the azobenzene anion captures one proton rapidly to produce the neutral species AH, while the spectrum observed at pH 14 is assigned to A-. Transient Spectra in 2-Propanol. The spectrum observed in neat i-PrOH was found to be identical with that in neutral aqueous solution, and is, therefore, assigned to the protonated radical AH. Since it was possible to observe the spectrum of A- in water at high pH, an attempt was made to observe it also in 2-propanol as solvent using (CH&CHO- as the base. The solutions were prepared by first dissolving Na metal in i-PrOH and then adding the azobenzene. The spectrum recorded has a main peak at 390 nm and a smaller one at 550 nm. This spectrum, which is quite different than that in neutral solution, is assigned to A- (Figure 4). It should be also noted that the same spectrum is obtained when starting from either syn- or anti-azobenzene. The pK for the protonation of A- in i-PrOH was also determined as shown in Figure 5. The inflection point in the best-fit curve corresponds to a concentration of 1.4 X M i-PrO-Na', from which the equilibrium constant for the process AH
+ i-Pro- 2 A-+ i-PrOH
(7)
was calculated to be K7 = 930. The results in aqueous solutions (Figure 3) showed an equilibrium constant K8 = 110 for AH
+ O H - 2 A- t H,O
(8 1
The Journal of Physical Chemistry, Voi. 81, No. 24, 1977
2290
P. Neta and H. Levanon
TABLE I: Rate Constants for Reduction of Azobenzene Reducing radical
Solute Solvent
J a
Flgure 4. Absorption spectrum of A- in 2-propanol recorded -50 /.LS after the pulse; [A] = 1 X lom4M, [LPrO-Na'] = 4 X lo-' M; (0) A,, (0)A,.
c
2 (cH,),CO-
t H+
(12)
which is known to be a stronger reductant,17 i.e., reaction 5 is replaced by the more rapid reaction
i-PrO-No']
(M)
Figure 5. Determination of the pKvalues of AH and of (CH3)&OH in neat 2-propanol. The values on the abscissa were determined by diluting an aliquot of the solutions with water and measurlng the pH: (0) absorption of azobenzene transient at 390 nm (left ordinate); (0) pseudo-first-order rate constant, k', for the reduction of azobenzene by the 2-propanol radical (right ordinate). In both experiments [A] was kept constant (1 X M).
As a first approximation, the values in the two solvents can be compared using the equations
PKAH - PKHaO If one neglects the effect of solvent and uses PKH~O = 15.75 and PKROH= 17.114the values of pKm calculated from eq 9 and 10 are similar. However, PKROHin the alcohol solution should be higher than that measured in water according to the relati~nshipl~ pKRoH(in ROH) - pKRoH(inH 2 0 ) = [DRtH
(3.3 c 0.3) x 10'' A, H,O' e (3.2 i: 0.3) x 10" A, H,Oa Baq CH,Oi x 109 A, H,Oa A, HzOu CH,OI X 109 (cH,),COH 4 x 108 Aa K O (cH,),CO2 x 109 A, H,O~ (CH,),COH 3 x 10' A, i-PrOH 2 X lo9 A, i-PrOHb (CH,),CO(CH,),CO2 X lo9 A, i-PrOHb 1 M NaOH. Containing 0.04 M i-PrO'Na'.
(cH,),COH
[
12ti;n
M-1s - l
From this value pKiH(in ROH) = 17. During these experiments it was noticed that the reduction of azobenzene by the i-PrOH radical became faster at high basicity. This result indicates that the radical dissociates to produce the anion
A
- ___-
Rate constant,
;%A
where D is the dielectric constant, ( r ) is the average radius of the ions, and n = 1 + z2(RO-) - z2(ROH) = 2. Assuming16 ( r ) x 3.5 A eq 11yields pKRoH(inROH) x 20. The Journal of Physical Chemistty, Vol. 81, No. 24, 1977
+ (CH,),CO-
+
A-t
(CH,),CO
(13)
The change in the observed rate of reduction can be used to estimate pKIP in i-PrOH. The results in Figure 5 (dashed line) show an inflection point at 7.7 X M i-PrO-Na+.. Applying the same considerations as discussed above for AH, pK(CH&oH(in ROH) is calculated to be -16. The pK of (CH3)2COHin water is Kinetics of Reduction of Azobenzene. From the above discussion it is clear that azobenzene undergoes oneelectron reduction by various species (e.g., reactions 4, 5, and 13) with a wide range of reactivities. The main reaction is the direct reduction by the electron (reaction 4) which is very fast. The rate constant for this reaction was determined for both syn- and anti-azobenzene in aqueous solutions containing 1 M base and 0.1 M methanol. The half-life for the decay of the e,; absorption at 600 nm was found to be -40 ps in the absence of A. Increasing the concentrations of A, or A, in the range of 2-12 /.LMgave correspondingly higher pseudo-first-order rates. A linear plot of the rates vs. concentration yielded k4 = (3.3 f 0.3) X 1O1O for A, and (3.2 f 0.3) X 1O1O M-l s-l for A,. Observation of the formation of A- at 380 nm gave similar results but the accuracy was limited by interference of the slow secondary electron transfer. The electron transfer from CH20- according to a reaction equivalent to .(13) was monitored by following the formation of the A- absorption at 380 nm. The rate constants were found to be 1 X lo9 M-l s-l for both isomers. Reduction by the 2-propanol radical was studied in both neutral and alkaline aqueous solutions. As mentioned above, the reduction by the dissociated form (CH3)ZCO(reaction 13) was found to be faster than that by the neutral form (CH3)&OH (reaction 5). Rate constants of h5 = 4 X lo8 and k13 = 2 X lo9 M-l s-l were determined (Table I). The rates of these reactions were also measured in neat i-PrOH yielding k6 = 3 x lo7and k13 = 2 X lo9M-l s-l. The higher value of k5 in water is probably due to the energy gain in the hydration of the H+ produced in this reaction. It should be noted that the second-order decay of the azobenzene radicals was sufficiently slow not to interfere with the formation and could be observed on the millisecond time scale.
Radicals Produced by Reduction of syn- and anfi-Azobenzene I
I
300
400
I
1
500
600
229 1
A(nrn1
Figure 6, Absorption spectrum of A- in THF: [A] = 1 X As, ( 0 )Aa.
A(nrn)
M; (0)
Identification of A-. The results presented so far show that the anion radical monitored in solution at room temperature is identical whether it is produced from A, or A,. The exact identity of this species cannot be elucidated from the experiments above. One limitation is the fact that the pK values for the protonation of A- in water and in 2-propanol are too high to, allow the experimental determination of the spectra of A- in the plateau region (see Figure 3 and 5). The spectra assigned to A- in Figures 2-4 represent a mixture with a small fraction of AH. The positions of the maxima, however, are not affected under these experimental conditions and are attributed to A-. The main peak is at 380 nm in water and at 390 nm in i-PrOH and in all cases it was identical for the syn- and anti-azobenzene. However, the equilibrium between Aand AH may form a channel by which A; and A, can interconvert. For example, in aqueous solutions such a mechanism is represented by
This mechanism can be eliminated by using an aprotic solvent. Experiments were, therefore, carried out in tetrahydrofuran and the spectra observed for A- produced from either syn- or anti-azobenzene were found to be identical (Figure 6 ) . The spectrum in THF has a maximum at 430 nm and only a shoulder at 500-540 nm. Red shifts in the main peak of 10 and 50 nm are thus observed in going from water to 2-propanol to THF. In view of the fact that the cis- and trans-stilbene anion radicals have different spectra, it seems unlikely that two isomeric azobenzene radicals exist with completely identical spectra. Furthermore, the spectra of the parent azobenzenes are also different for the two isomers.'~~If A- exists in one configuration only, one can expect radiolytic isomerization of azobenzene via production of Awhich is then oxidized by the solvent positive ion back to A. Vacuum sealed solutions of either A, or A, in THF at room temperature were prepared and their optical spectra were monitored before and after pulse irradiation. When starting with A, the absorption in the spectral range of 400-500 nm was found to decrease considerably. This indicates that A, is converted into A,. Conversion in the opposite direction when starting with A, should result in an increase in the absorption in that region. However, no such increase could be experimentallyobserved. Since part of the azobenzene was destroyed to form products with new absorption in the UV, observation of the isomerization
Figure 7. Absorption spectra of A; and A; in MTHF glass at 77 K. The spectra before irradiation were taken for comparison. A, was irradiated for 15 min and A, for 45 min. The dose rate was ~3 X 1017 eV g-' min The optical path was 3 mm.
-'.
below 400 nm was less sensitive but generally confirmed the above results. It appears that the anion radical produced from either isomer yields upon oxidation the anti isomer predominantly:
The intermediate cannot, therefore, be a mixture of A; and A;. Although some intermediate configuration or a free rotating structure cannot be ruled out, it is more likely that in solution the observed A- is in its anti form. The assignment of the ESR proton hyperfine splittings18to A; supports this conclusion. If the intermediate observed is indeed A , the conversion of A; into A, is too rapid to observe at room temperature. Experiments were, therefore, carried out at liquid nitrogen temperature in MTHF glass. Frozen solutions of A, and A, were irradiated and the optic$ spectra were compared (Figure 7). The spectrum of A; in the glass is identical with that observed in THF solutions at room temperature. The spectrum obtained using A, has its maximum also at 430 nm but the intensity per unit dose is almost an order of magnitude lower. These findings, combined with the results obtained with THF solution at room temperature, lead to the conclusion that under the latter conditions A; is converted into A; very rapidly (
